A ceramic membrane separation apparatus of the type which is adapted to be submerged in treatment water contained in a water treatment vessel is well known as a solid-liquid separator for water treatment such as water purification and waste water treatment of sewage water, night soil and industrial waste water, or for concentration of sludge, separation of bacteria, desalination of sea water, or like treatments. Such ceramic membrane separation apparatus, for example, comprises a plurality of stacked membrane modules each having a plurality of tubular ceramic membrane filters, and filtrate suction pipes communicating with the inside of the tubular ceramic membrane filters. In this ceramic membrane separation apparatus, treatment water contained in the water treatment vessel is filtered through the ceramic membrane filters by the water pressure of the treatment water in the water treatment vessel or a suction pressure generated by a suction means connected to a filtrate suction pipe line, and the filtrate thus filtered is removed from the water treatment vessel. When scale, slime and other substances deposited on the surface of and inside of the ceramic membrane filters deteriorate the filtering performance of the apparatus during a prolonged operation, the ceramic membrane filters are chemically cleaned by immersing the entire membrane separation apparatus in a chemical agent in the water treatment vessel after the water is drained therefrom, or in an another vessel outside the water treatment vessel.
Though the chemical cleaning is indeed a highly effective cleaning method, deposited substances can otherwise be removed merely by washing with water to some extent. Therefore, the chemical agent is not necessarily required for every cleaning, and is undesirable in terms of cost.
A method for back-washing membrane filters is also known in which a chemical agent is forcibly supplied to filtrate passages inside the membrane filters and then permeated through the membrane filters from the filtrate passages to clean the membrane filters. In this method, however, the chemical agent flows through the filtrate passages in a state of plug flow and hence is liable to remain unreacted in the middle portion of the membrane filters. In addition, there is a tendency to use an excessive amount of an oxidizing agent such as sodium hypochlorite to enhance the cleaning effect. As a result, residual chemical agent in the filtrate passages comes out along with the filtrate when a filtering operation is resumed. The chemical agent contaminating the filtrate may affect adversely in the later steps of the water treatment process.